This invention relates to a method of tightly joining two semiconductor substrates together by using a simple liquid to wet the substrate surfaces to be brought into contact with each other.
In the manufacture of semiconductor devices it is an essential process to form with high accuracy and good reproducibility a plurality of regions with different impurity concentrations within a semiconductor substrate. Thus far, this purpose is accomplished by selectively using well known techniques, such as impurity diffusion, ion implantation and epitaxial growth. However, in the case of forming a very thick layer having a specific conductivity and a thickness of hundreds of microns, it is impracticable to achieve the object by the usual diffusion or implantation technique, and it takes a very long time to form such a thick layer by epitaxial growth.
To obviate the difficulty of forming such a thick layer in a substrate, it is conceivable to join two semiconductor substrates different in impurity concentration with an adhesive. However, adhesives useful for this purpose usually comprises a polymeric material, which prevents establishment of ohmic contact between the two substrates joined together and causes intrusion of a large amount of foreign matter into the substrate obtained by the joining.
In some devices using semiconductive materials, such as pressure sensors having a silicon diaphragm, it is possible to bond a silicon part to a substrate with a nonconductive adhesive such as a low melting point glass. However, the resultant joints are not always sufficient in strength of adhesion and, when thermally stressed, are liable to break because of a significant difference of the expansion coefficient of the adhesive glass from that of silicon.
JP-A No. 60-51700 shows a method of joining two silicon substrates together, comprising the steps of polishing the opposite surfaces of the substrates, rendering the polished surfaces hydrophilic by treating with a suitable liquid such as a mixture of an acid and hydrogen peroxide, and, after drying, bringing the treated surfaces into tight contact with each other. However, by this method accurate alignment of the two substrates is difficult, because once the subtrates are brought into contact with each other, there occurs strong cohesion which prevents correcting location by sliding one of the substrates on the other. As another matter of inconvenience, the hydrophilic surfaces of the substrates must be left exposed until the joining operation and hence are liable to be damaged or contaminated. Besides, the joining operation needs to be performed in vacuum because if air remains at the interface between the joined substrates the nitrogen and oxygen atoms, which have unfavorable influences on the contacting surfaces, cannot easily be diffused even by making a heat treatment at about 1000.degree. C.